A recent in vivo proton magnetic resonance spectroscopy (1H MRS) study by the applicants has documented a 36% decrease in mean cortical levels of glutathione (GSH), the most abundant and important antioxidant in living tissue, in chronic fatigue syndrome (CFS) compared to healthy control subjects. This finding, along with prior credible reports of increased blood markers of oxidant damage in CFS, strongly implicates increased oxidative stress in the pathophysiology of the disorder, and suggests the viability of treatment strategies based on restoring cortical GSH levels to normalize oxidative stress. However, direct dietary supplementation of GSH has not proved viable in increasing cortical levels of the antioxidant due to its poor permeability through the blood-brain barrier (BBB) or the membranes of most cells, including neurons. On the other hand, since the bioavailability of cysteine - a common amino acid with favorable BBB and membrane transport properties - is rate-limiting in the GSH synthesis pathway, there is a great deal of interest in investigating the non-toxic derivative of this amino acid, N-acetylcysteine (NAC), as a synthetic precursor that can be supplied through dietary means to spur in situ elevation of brain GSH. These considerations suggested the main objectives of the present Exploratory/Developmental (R21) project, which are: (a) to investigate whether 4 weeks of daily supplementation with 2000mg of NAC will elevate cortical GSH, as measured in vivo by 1H MRS, in 15 patients with CFS compared to 15 matched healthy control subjects and to baseline; (b) to determine whether GSH elevations due to NAC, if any, would normalize oxidative stress in CFS, as assessed by changes in the levels of established makers of oxidative stress in body fluid samples from all participants; and (c) to explore whether normalization of levels of cortical GSH and oxidative stress markers will be associated with amelioration of CFS symptoms. If successful, this study has the potential to (i) contribute further evidence in support of the emerging oxidative stress model of CFS, (ii) to establish whether NAC supplementation leads to in vivo elevations of cortical GSH, and (iii) to provide preliminary evidence on whether the intervention ameliorates CFS symptoms, which would justify larger studies of NAC or other GSH precursors as potential neuroprotective treatments for CFS.